1. Field of the Invention
The present invention relates to a process for the production of a laminated, enamelled glazing comprising at least two glass sheets, whereof at least one is at least partly coated with at least one enamel coating on its face directed towards the interior of the laminate.
The invention also relates to an enamel composition advantageously usable in the aforementioned process, as well as to the products obtained according to said process.
2. Description of the Prior Art
Enamels are well known in the art and are in particular used on glass substrates such as glazings for vehicles or buildings. Inter alia they make it possible to form borders, decorative layers or protective layers. Thus, in car glazings, enamels are used for hiding the collective strips of heating systems or for hiding and protecting the actual heating systems. They are more particularly used for protecting adhesive beads used for the installation of the glazings in the vehicle body openings against light radiation and in particular UV radiation, whilst at the same time as masking the said beads.
The enamel used for such applications is generally formed from a powder incorporating a glass frit including pigments and a medium, also known as a vehicle or vector generally comprising a binder ensuring the cohesion of the enamel and the temporary adhesion of said enamel to the glass at the time of deposition, and an organic diluent or solvent making it possible to give the necessary viscosity for applying the enamel to the substrate.
In general terms, the enamel is deposited by screen printing. Thus, using this procedure, it is known to deposit the enamel on the substrate, dry the thus obtained layer in order to permit the elimination of the diluents in the enamel, optionally deposit other layers which also undergo drying, prior to the heat treatment of all the enamelled layers at a high temperature in order to vitrify the enamel and obtain the final covering.
In the case of laminated glazings comprising at least two glass sheets, it is known to deposit the enamel by screen printing on the face of one glass sheet which is to be turned towards the outside of the glazing, prior to the drying and then firing of said enamel, the firing of the enamel taking place during the simultaneous bending of the glass sheets of the glazing for bent, laminated glazings. A simultaneous bending of the glass sheets of a laminated glazing superimposed in their final order and with simultaneous firing of the enamel layer or layers located on at least one of the sheets is only possible when the enamel is intended to be located on the outside of the laminated glass. However, in the opposite case, where it is wished to provide a laminated glazing with an enamel layer on a substrate face to be in contact with another substrate of the laminated glazing, i.e. a face of the substrate turned towards the inside of a laminated glazing, the firing of the enamel during the simultaneous bending of the superimposed substrates in their final order causes numerous problems. Thus, the enamel incorporates components, particularly organic components, which, at the firing temperature, give off reaction products in gaseous form, which cannot be evacuated out of the space between the substrates. There is also a considerable risk of the substrates sticking to one another by means of the enamel during the melting of the latter.
In order to obviate these problems, it is known to bake the enamel on the planar substrate prior to bending. Thus, DE-A3,920,573 proposes subjecting the still planar, enamelled substrate to firing at a high temperature (approximately 600.degree. C.), then allowing the substrate to cool before superimposing a second substrate and simultaneously bending the two substrates. This process makes it possible to solve the problems associated with the reaction gases, sticking being avoided by the use of separating or parting agents between the two substrates, but this involves the complying with several requirements. Thus, the enamelled substrate must remain flat and must have no internal stress liable to cause deformations during bending. The transportation of the substrates on conveying rollers can lead, for example, to minor deformations which are still adequate to ensure that the resulting glazings have inadequate optical properties. Moreover, a high temperature treatment prior to bending is expensive and may give rise to irreversible optical faults.
Another process described in application DE 4,201,275.9 proposes carrying out a prebending and a simultaneous baking of the enamel, but reversing the order of the substrates, so that the enamel-covered face is directed towards the outside. Following the cooling of the thus prebent substrates, the position of the substrates is reversed and said substrates are bent on their final shape, the enamelled substrate on this occasion having its enamel-covered face directed towards the inside of the laminated glazing. However, this process makes it necessary to have a supplementary bending stage, as well as a supplementary cooling stage, accompanied by a reversal of the substrates, so that costs are raised.
Therefore a first object of the invention is to obviate the problems of enamelling within a laminated glazing by proposing a simple and effective solution making it possible to obtain products of adequate quality at a lower cost and with improved productivity.
More particularly, the object of the invention is to develop a process for the production of a laminated, enamelled glazing, in combination with an appropriate composition, said process making it possible to simultaneously bend the substrates of the laminated glazing superimposed in their final order and a firing, during the bending, of the enamel used, when said enamel is located on one face of at least one of the substrates to be turned towards the inside of the laminated glazing, said process being simpler and having better performance characteristics than the aforementioned, known processes.